We have argued that reverberation mapping provides a unique probe of the inner structure of AGNs. The reverberation technique has been very successful in determining the BLR sizes and black hole masses in 35 AGNs. The masses obtained are accurate to about a factor of 3, based on the observed scatter in the AGN MBH - * relationship. The accuracy of these masses is fundamentally limited by unknown geometry and kinematics of BLR. We have also argued that it is possible to obtain complete, high-fidelity velocity-delay maps of the strong ultraviolet lines in relatively nearby, moderately luminous AGNs with HST, and we specifically argue that this can be done with high confidence of success for NGC 5548 with one HST orbit per day for a period of no longer than 200 days.
We are grateful for support by the National Science Foundation (grant AST - 0205964) and PPARC.